1. Field of the Invention
The present invention relates to a digital channel selecting apparatus. More specifically, the present invention relates to a digital channel selecting apparatus, wherein a plurality of capacitance elements are coupled in parallel, each constituting a portion of a tuning circuit, such that each capacitance element is individually coupled to a corresponding switching device which is operable responsive to a digital control signal, whereby the total capacitance value associated with the tuning circuit is varied to achieve channel selection.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an outline of a tuner or a channel selecting apparatus 1 for use in a typical television receiver. As well known, the tuner 1 comprises an input tuning circuit 2, an inter-stage tuning circuit 3, a local oscillator 4 and a mixer 5 for mixing the high frequency signal from the inter-stage tuning circuit 3 with the local oscillation signal from the local oscillator 4 to provide an intermediate frequency signal through superheterodyne detection.
In a chennel selecting operation by means of the tuner 1, the input tuning circuit 2, the inter-stage tuning circuit 3 and the tuning circuit included in the local oscillator 4 must be varied to a desired tuning frequency determinable depending on the channel to be selected. In general, a typical scheme so far adopted for varying the tuning frequency of a tuning circuit is to vary a reverse bias voltage to be applied to a voltage controlled variable capacitance diode coupled in the tuning circuit, thereby to vary the capacitance across the diode and hence to vary the tuning frequency of the tuning circuit.
Recently, a variable capacitance device comprising a plurality of series connections coupled in parallel with each other each including a capacitance element and a switching diode was developed. In using such variable capacitance device, the same is connected such that the total capacitance is varied by selectively applying a digital control signal to each of the switching diode, thereby to select a desired capacitance element. Thus, such variable capacitance device could be advantageously utilized for the purpose of implementing a tuning circuit. FIG. 2 shows a schematic diagram of such a tuning circuit that could be implemented using the above described variable capacitance device comprising a plurality of series connections coupled in parallel with each other each including a capacitance element and a swtiching diode. It is needless to say that such a tuning circuit as shown in FIG. 2 may be employed as the input tuning circuit 2, the inter-stage tuning circuit 3, and the tuning circuit included in the local oscillator 4 shown in FIG. 1, for example.
FIG. 2 shows an example of a tuning circuit for use in a very high frequency or VHF application. Thus, the VHF tuning circuit shown comprises series connected coupling capacitors 9 and 10, an inductance coil L coupled in parallel between these couping capacitors 9 and 10 and the above described variable capacitance device C coupled in parallel between these capacitors 9 and 10. The variable capacitance device C comprises a plurality of capacitance elements C1, C2, C3 . . . Cn-1 and Cn, and a corresponding plurality of switching diodes D1, D2, D3, . . . Dn-1 and Dn, each connected in series with the corresponding one of the above described capacitance elements C1, C2, C3, . . . Cn-1 and Cn, respectively, thereby to constitute a corresponding plurality of series connections in parallel with each other, each including a capacitance element and a switching diode. The junctions of the respective series connections are coupled through corresponding resistors R1, R2, R3, . . . Rn-1 and Rn to output terminals T1, T2, T3, . . . Tn-1 and Tn, respectively, of a digital signal generator 7. The switching diodes D1, D2, D3, . . . Dn-1 and Dn are each responsive to the corresponding individual digital control signal from the digital signal generator 7 to be rendered conductive, whereby the capacitance element coupled to the switching diode now rendered conductive is rendered effective or selected as a capacitance to constitute a portion of the tuning circuit as desired. For the purpose of facility of understanding, assuming that only the switching diodes D1 and D2 are selected as a function of the control signals from the digital signal generator 7, the total capacitance of the variable capacitance device C can be simply calculated as (C1+C2). Thus, the total capacitance of the variable capacitance device C can be varied in a digital manner or a stepwise manner as a function of the digital control signal.
FIG. 3 shows an example wherein the above described variable capacitance device C is utilized coupled to a 1/2 wave length resonance type tuning circuit in a ultra high frequency or UHF application. Again it is pointed out that the above described variable capacitance device C could be advantageously utilized as a portion of a tuning circuit in a UHF application as well. As well known, the fundamental circuit of a 1/2 wave length resonance type tuning circuit comprises a resonance conductor both ends of which are each coupled through a capacitance device to the ground. It would be seen that in the FIG. 3 example a capacitance device coupled to one end of the resonance conductor has been replaced by the above described variable capacitance device C as discussed with reference to FIG. 2. More specifically, one end of a resonance conductor L0 serving as an inductance element is coupled through the above described variable capacitance device C to the ground, while the other end of the resonance conductor L0 is coupled through a fixed capacitance device C0 to the ground.
Similarly, FIG. 4 shows an example of a 1/4 wave length resonance type tuning circuit employing the above described variable capacitance device C for use in a UHF application. Again, it is pointed out that FIG. 4 merely shows an example wherein the above described variable capacitance device C has been shown as theoretically applicable to the said application. The fundamental circuit of a 1/4 wave length resonance type tuning circuit comprises a resonance conductor, one end of which is directly grounded and the other end of which is coupled through a capacitance device to the ground. The tuning circuit shown in FIG. 4 is shown as the other end of the resonance conductor being connected through the above described variable capacitance device C to the ground in place of an ordinary capacitance device.
It is pointed out that although various examples of application of the above described variable capacitance device C as a tuning circuit component have been shown in FIGS. 2 through 4 these were depicted as merely theoretically applicable. In practicing such examples in actuality, however, various problems would be encountered, as to be more fully discussed in the following. At the outset, with such a digital channel selecting apparatus as discussed with reference to FIGS. 2 through 4, the tuning frequency can be only adjusted in a stepwise manner with a small frequency change .DELTA.f for each small change .DELTA.C of the capacitance attained by the variable capacitance device, which could leave a slight frequency deviation .DELTA.f0 with respect to a desired or normal frequency f0, even when a tuning state is established. No problem would occur within the range where such frequency deviation or drift is allowed. Thus, the minimum required capacitance change .DELTA.C0min of the above described variable capacitance device is determined with respect to the maximum allowable frequency deviation or drift .DELTA.f0max which is determinable from the stand point of the tuning circuit operation. It is pointed out that the above described minimum required capacitance change .DELTA.C0min indicates a width with which the capacitance of the variable tuning capacitance device is varied in a stepwise manner or in a digital manner. Accordingly, the minimum unit of the respective capacitance of a plurality of tuning capacitance elements coupled in parallel with each other of the variable capacitance device must be smaller than at least the above described minimum required capacitance change .DELTA.C0min. The reason is that the capacitance of each of a plurality of tuning capacitance elements coupled in parallel with each other of the variable capacitance device is determined as a total sum of the capacitance value of each elements. Thus, the broader the frequency region to be covered by the tuning circuit, the smaller the minimum required capacitance change .DELTA.C0min for the above described maximum allowable frequency deviation .DELTA.f0max. The reason is that since the parallel resonant frequency f is generally determined by the equation ##EQU1## assuming that resistance of a coil is neglected, the higher the frequency region of the resonance frequency f of the tuning circuit, the smaller the capacitance of the variable capacitance device and accordingly the smaller the required capacitance change .DELTA.C0 for the allowable frequency deviation .DELTA.f0 in such a higher frequency region of the resonance frequency f. Thus, the required capacitance change for the allowable frequency devitation in the maximum frequency of such a higher region becomes the minimum required capacitance change .DELTA.C0min.
Since the minimum unit of the respective capacitance of a plurality of tuning capacitance elements coupled in parallel with each other of the variable capacitance device must be smaller than at least the minimum required capacitance change .DELTA.C0min, as thus described, the broader the frequency region to be covered by the tuning circuit, the smaller the minimum required capacitance change .DELTA.C0min, with the result that the respective capacitance elements constituting the variable capacitance device would be of an extremely small capacitance value. As a result, implementation of a channel selecting apparatus so as to cover all the channels using a plurality of tuning capacitance elements thus selected increases the number of tuning capacitance elements, which makes fabrication of the apparatus difficult. In addition, assuming that such capacitance elements are fabricated in an extremely small capacitance value by means of a thick film or thin film process, there is a limit to the resolution of the capacitance, which causes a problem as compared with a case where a conventional channel selecting apparatus using a voltage controlled variable capacitance diode exhibiting a linear characteristic is employed.
In order to more specifically describe such a problem, the tuning circuit shown in FIG. 3 is more specifically considered. For facility of explanation, the equivalent circuit of the FIG. 3 example is shown in FIG. 5. Referring to FIG. 5, a variable capacitance C denotes a composite capacitance of the capacitance of the capacitance elements C1, C2, C3, . . . Cn-1 and Cn shown in FIG. 3 and the resonance conductor L0 is shown as having the total length l. Referring to the FIG. 5 equivalent circuit, assuming the characteristic impedance of the tuning circuit to be Z0 and the frequency to be f, then the total capacitance C.sub.D of the tuning circuit may be expressed by the following equation. ##EQU2##
Now assuming that l=0.006 m, Z0=215 .OMEGA., CO=30 pF and Cp=5.5 pF, the relation between the required capacitance and the frequency and the relation between the frequency deviation .DELTA.f (in this case, the deviation of .+-.0.5 MHz) and the required capacitance change .DELTA.C are shown in the following Table I. It is pointed out that the frequency deviation .DELTA.f has been selected to be .+-.0.5 MHz, because the maximum frequency deviation that can be corrected by means of an automatic frequency control in an ordinary television receiver is approximately .+-.0.5 MHz.
As seen from Table I, the minimum capacitance change .DELTA.C0min in case of the frequency deviation .+-.0.5 MHz is approximately 0.01 pF in case of reception of the signal 894.15 MHz. Accordingly, it is required that the minimum capacitance value of the respective capacitance elements C1, C2, C3, . . . Cn-1 and Cn be smaller than 0.01 pF and nevertheless a combination of such capacitance elements must cover the maximum value of the capacitance C.sub.D obtained by the above described equation, i.e., approximately 40 pF for the frequency 510.15 MHz. However, it would be appreciated that in order
TABLE I ______________________________________ Capacitance Change (pF) in case of Frequency f (MHz) C.sub.D (pF) Deviation .+-. 0.5MHz ______________________________________ 510.15 39.607 + 0.22038 - 0.22232 542.15 23.570 + 0.13377 - 0.13470 582.15 20.176 + 0.8168 - 0.08212 622.15 14.813 + 0.05467 - 0.05493 662.15 11.111 + 0.03895 - 0.03910 702.15 3.415 + 0.02901 - 0.02911 742.15 6.374 + 0.02234 - 0.02241 782.15 4.781 + 0.01766 - 0.01772 822.15 3.509 + 0.01427 - 0.01431 862.15 2.472 + 0.01173 - 0.01175 894.15 1.778 + 0.01013 - 0.01015 ______________________________________
to vary in a stepwise manner or in a digital manner the capacitance with a variation width of 0.01 pF, it is extremely difficult to implement the capacitance elements C1, C2, C3, . . . Cn-1 and Cn so as to satisfy the total capacitance of 40 pF. For the purpose of description of the present invention in the specification, a stepwise or digital variation of the capacitance with a variation width of say 0.01 pF is defined as "a resolution of 0.01/step". Even if such a voltage controlled variable capacitance device including a plurality of capacitance elements coupled in parallel with each other could be implemented such that the capacitance of each capacitance element is as small as 0.01 pF, the total capacitance value of 40 pF required as described above entails an increased number of capacitance elements and hence an increased number of bits of the digital control signal, which makes undesirably complicated the channel selecting apparatus.
In order to solve the above described problems, it could be considered that a variable capacitance range of the variable capacitance device is confined to a limited narrow range and a broader frequency region is covered by selectively switching an additional reactance device of the tuning circuit, thereby to cover the desired range of the frequency using such a variable capacitance device either with or without such an additional reactance device. A typical prior art for selectively switching such additional reactance device in a tuning circuit has been known in a circuit configuration where a voltage controlled variable capacitance device combined with an inductance for achieving a relatively narrow capacitance variation region is used to cover a high frequency region, without an additional inductance element, and to cover a lower frequency region, with such additional inductance element, such that a variation range is selectively shifted among the higher and lower frequency regions. However, such a prior art system for selectively shifting a relatively narrow variation range among the higher and lower frequency regions merely employs a voltage controlled variable capacitance device of a given narrow variation capacitance range and such a prior art system is different from the tuning circuit as discussed with reference to FIGS. 2 through 4, wherein a different combination of the respective capacitance elements of a variable capacitance device C is adapted to cover all the frequency regions of both the high and low frequency regions, for example. It is pointed out that the present invention is directed to an improvement in a tuning circuit using a variable capacitance device C as discussed with reference to FIGS. 2 through 4, wherein a combination of the respective capacitance elements is selectively switched to cover all the frequency regions of both the high and low frequency regions, for example, by different combination of these capacitance elements.